Cable modem usage to send and receive digital data is on the rise with the buildout of high bandwidth cable TV systems with spare bandwidth in which to send high speed digital data such as internet traffic. An emerging standard for cable modems is currently under development which defines cable modem systems wherein mixed mode upstream transmissions can be received. The reason this is important is that legacy cable systems exist such as DOCSIS 1.0 and 1.1 that deliver time division multiplexed (hereafter TDMA) digital data bursts over cable television media. However, there are significant advantages to using code division multiplexing, and especially synchronous code divsion multiplexed (hereafter SCDMA) bursts. Specifically, the use of SCDMA provides significant “code gain” to suppress the negative effects caused by both narrowband interference and Gaussian noise suppression. In the upstream where many cable modems (CMs) in homes and businesses transmit digital data to a cable modem termination system (CMTS) at the cable plant headend, there are many sources of interference which can leak into the upstream signals. These sources include echoes from improperly terminated cable outlets, ignition noise, radio frequency interference generated by transmitters and household appliances, etc. In some prior art cable modem distributed digital data delivery systems, these noise problems have been so intractable, that they led to the ultimate failure of the system to fully live up to its promised potential.
The upstream transmissions from the cable modems are in a FDMA/TDMA or FDMA/SCDMA burst modulation format. That is, frequency division modulation is used to create a plurality of logical channels at different center frequencies. Inside each FDMA channel, time division multiplexing (TDMA) or synchronous code division multiplexing (SCDMA) is used to create logical subchannels. In the legacy DOCSIS systems, the timeslots used by the TDMA process are called minislots. The SCDMA bursts also use minislots, but the multiplexing is actually accomplished in a two dimensional space having codes as one axis, and spreading intervals within minislots as the other axis.
Permission to transmit is requested by a CM in the form of an upstream bandwidth request. Such requests are sent autonomously by CMs having data to send upstream, but such requests are only sent during request intervals defined in the upstream by MAC messages transmitted downstream. If there is a collision between upstream requests, the CM that transmitted the request does not receive any reply in the downstream MAP messages that define when bursts may be transmitted for various Service Identifiers (SIDs). The downstream MAP messages are broadcast and not directed to any particular CM. The CM only knows when it has an authorization to transmit a burst when a SID it owns is included in a MAP message. Assuming not collision on the request, the CMTS responds with UCD and MAP messages. These messages define the burst parameters and the interval during which the burst may be sent. Specifically, the MAP message defines the logical channel and subchannel(s) to use (when the burst may start and how long it may be and on which frequency channel) and the burst type in terms of an IUC identifier. There were originally 6 different burst types defined in DOCSIS 1.0, but the class of advanced PHY layer receivers according to the genus of the invention will be able to receive 15 different burst types. The UCD messages define the burst parameters such as the type of multiplexing (TDMA or SCDMA), the type of modulation to use, the symbol rate and error correction encoding to use, interleaving depth, whether Trellis encoding is on or off, other parameters, and any neccessary correction to power level at which to transmit the burst so as to keep the received power at the CMTS within a gain controlled range of the receiver for to not saturate the A/D converter and use its full dynamic range. All CMs on the same logical channel or subchannel use the assigned symbol rate in the UCD message in the prior art systems.
The modulation modes are controlled by the UCD message and include 16 QAM for TDMA channels, Gray coded QPSK and 8 QAM to 64 QAM for TDMA and SCDMA channels and Trellis encoded QPSK and 8 QAM through 128 QAM for SCDMA channels. The symbols transmitted in each mode and the mapping of input bit to the I and Q constellation must be as defined in a predetermined table(s) having specified mappings for each type of modulation. Thus, CMTS receivers that are fully compatible with this emerging physical layer standard (which has no assigned name or specification number yet) for broadband digital data delivery over cable TV plants must be able to receive either SCDMA or TDMA bursts at different symbol rates and using different forms of modulation and different error correction encoding. For example, to be fully compatible with the emerging system which has legacy DOCSIS TDMA only transmitters in some CMs, a receiver should be able to receive QPSK and 16 QAM modulation for TDMA bursts. To be fully compatible, such a receiver must be able to receive QPSK, 8 QAM, 16 QAM, 32 QAM, 64 QAM modulations for TDMA and SCDMA bursts if the system has some cable modems that transmit using these forms of modulation and multiplexing. Further, to be fully compatible with SCDMA bursts from the most modern cable modems, the receiver must be able to receive QPSK or M-ary QAM including 8 QAM, 16 QAM, 32 QAM, 64 QAM and 128 QAM Trellis Coded modulations (hereafter TCM). In addition to be fully compatible, such a receiver would have to receive bursts transmitted at 160, 320, 640, 1280, 2560 and 5120 kilosymbols per second (ksym/sec) and using different forms of Trellis encoded modulation or other predetermined encoding to add error correction bits.
In addition, to be fully compatible with such a system, the receiver in the CMTS must be able to perform symbol timing (clock recovery), carrier recovery and tracking, burst acquisition and demodulation in the radio frequency section. In addition, to be fully compatible, the receiver must be able to provide an estimate of burst timing relative to a reference edge to provide a timing offset to assist in the DOCSIS TDMA based ranging process. In addition, to be fully compatible, such a receiver must provide a frequency offset and an estimate of signal power and should be able to participate in adaptive equalization with the cable modems to mitigate the effect of echoes in the cable plant, narrowband interference and group delay.
The cable modem transmitters in such a system do some signal processing to interleave data to minimize burst errors, scramble it for privacy or to break up long runs of 0s or 1s, encode it with error correction bits (programmable Forward Error Correction encoding) and multiplex it with either TDMA or SCDMA multiplexing. TDMA bursts are timed by the cable modems so that the center of the last symbol of the previous burst is separated by a guardband of at least 5 symbol times plus the maximum timing error from the center of the first symbol of the preamble of the next burst.
To be fully compatible with such a system, the CMTS receiver must be able to do the inverse processing of all the signal processing and multiplexing that the transmitters did. The signal processing function in the CMTS receiver, to be fully compatible, also must be able to support the DOCSIS ranging process by providing an edge-timing reference in an upstream gap opened by the CMTS. This gap is used by the cable modem transmitters (hereafter the CMs) to perform DOCSIS ranging. The signal processing function in the CMTS receiver, to be fully compatible, also must provide a gating-enable signal to the demodulators to activate the burst acquisition process during the Minislots assigned to any particular burst.
Further, such a mixed mode CMTS receiver must be able to do either coherent detection to receive advanced physical layer TDMA and SCDMA bursts (hereafter sometimes referred to as advanced PHY bursts) or coherent detection/differential decoding so as to be capable of receiving DOCSIS 1.0 or 1.1 bursts.
Thus, a need has arisen for a head end receiver for use in the CMTS which is capable of receiving both advanced PHY SCDMA, and TDMA bursts as well as DOCSIS 1.0 or DOCSIS 1.1 bursts with different modulation types and different symbol rates.